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8.3: Thermochemistry of Substitution Reactions

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    Ionic or polar reactions of alkyl halides rarely are observed in the vapor phase because the energy required to dissociate a carbon-halogen bond heterolytically is almost prohibitively high. For example, while the heat of dissociation of chloromethane to a methyl radical and a chlorine atom is \(84 \: \text{kcal mol}^{-1}\) (Table 4-6), dissociation to a methyl cation and a chloride ion requires about \(227 \: \text{kcal mol}^{-1}\):

    Top: C H 3 bonded to C L gas gets cleaved and goes to C H 3 radical gas plus C L radical gas with delta H of positive 84 K cal. Bottom: C H 3 bonded to C L gas gets cleaved and goes to C H 3 cation gas plus C L anion gas with delta H of positive 227 K cal.

    However, the heat of ionic dissociation of methyl chloride in aqueous solution is estimated to be \(63 \: \text{kcal}\), and while this reaction is still substantially endothermic, it requires about \(227 - 63 = 164 \: \text{kcal}\) less energy than in the gas phase:

    C H 3 bonded to C L aqueous goes to C H 3 cation aqueous plus C L anion aqueous with delta H of positive 63 K cal.

    The reason is that ions are much more stable in water than in the gas phase; for example, the transfer of a chloride ion from the gas to water is exothermic by \(-85 \: \text{kcal}\). The \(\Delta H^\text{0}\) value for the corresponding transfer of a methyl cation, \(CH_3^\oplus\), is not known with certainty, but is about \(-80 \: \text{kcal}\). These ionic solvation energies are clearly large. In contrast, the \(\Delta H^\text{0}\) for solution of methyl chloride in water is small (about \(1 \: \text{kcal}\)). We can use these data to calculate the heat of ionic dissociation of chloromethane in water:

    C H 3 C L gas goes to C H 3 cation gas plus C L anion gas with a delta H of 227 K cal squared. C H 3 C L aqueous goes to C H 3 C L gas with a delta H of plus 1 K cal. C H 3 cation gas goes to C H 3 cation aqueous with a delta H of negative 80 K cal. C L anion gas goes to C L anion aqueous with a delta H of negative 85 K cal. Net: C H 3 C L aqueous goes to C H 3 cation aqueous plus C L anion aqueous with a delta H of positive 63 K cal.

    Thermochemical data for the solvation of ions as used in the preceding calculations are difficult to measure and even to estimate. Therefore this kind of calculation of \(\Delta H^\text{0}\) for ionic reactions involving organic molecules in solution usually cannot be made. As a result, we have considerably fewer possibilities to assess the thermodynamic feasibility of the individual steps of polar reactions in solution than we do of vapor-phase radical processes. Bond energies are not of much use in predicting or explaining reactivity in ionic reactions unless we have some information that can be used to translate gas-phase \(\Delta H^\text{0}\) values to solution \(\Delta H^\text{0}\) values.

    \(^2\)Calculated from the following data:

    Top: C H 3 C L gas goes to C H 3 radical plus C L radical gas with a delta H of positive 84 K cal. Labeled bond-dissociation energy. Middle: C H 3 radical gas goes to C H 3 cation gas plus an electron with a delta H of positive 226.7 K cal. Labeled ionization potential from mass spectral studies. Bottom: C L radical gas plus electron goes to C L anion gas with a delta H of negative 83.2 K cal. Labeled electron affinity from mass spectral studies. Total delta H is positive 227 K cal.

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

    This page titled 8.3: Thermochemistry of Substitution Reactions is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.